1. Field of the Invention
This invention relates to distributors which provide substantially uniform fluid flow through cells. These devices are useful in fuel cells, electrochemical cells, dialyzers, electrodialyzers, and other apparatus of that type and are particularly useful in dialyzers, especially plural cell electrodialyzers.
2. Description of the Prior Art
Electrodialysis is a process for the partial separation of components of an ionic solution by means of electrical forces which selectively drive ions through membranes. Thus electrodialysis is a selective transport process in which salt or solvent is separated from a feed solution without any component of the solution changing state. As compared with processes such as distillation and freezing, which accomplish separation by changing the state of the solvent, electrodialysis generally requires less energy. The process is coming into increasing use in such applications as production of potable water from sea water or brackish water by desalination, production of salt from sea water, demineralization of organic matter, and preparation and purification of inorganic chemicals. Each of the above-mentioned applications involve contacting one or more solutions with permselective membranes in compartments or cells. A complete unit may comprise up to 2000 or more cells, generally limited by electrical power requirements.
Within each cell of an electrodialyzer, a spacer separates adjacent membranes and determines the fluid flow pattern therein. Generally the spacer configuration is such as to provide either tortuous-path flow or sheet flow. Tortuous-path flow can provide a long residence time of the fluid in each cell. It is accomplished with a spacer in which a cutout pattern provides a sinuous path for the fluid. A drawback of the tortuous-path flow system is that it necessitates a large pressure drop across the cell. In addition, providing a tortuous path requires a substantial spacer area, which reduces the effective membrane area.
Whereas tortuous-path flow is a form of series flow within a cell, sheet flow is parallel flow; i.e., not all fluid elements follow the same path within the cell. Typically, sheet flow is accomplished with a spacer screen which provides a multitude of alternate paths from the inlet to outlet port. Although the screen provides desirable turbulence in the fluid flow, it does not provide uniform flow across the (substantially rectangular) area of the cell. Instead, flow is greater within a substantially oblong area between the inlet and outlet ports. Fluid in the cell which is outside the oblong area is relatively stagnant and does not contribute significantly to the dialysis. Consequently, there is a loss of efficiency.
Another problem afflicting electrodialysis cells of the prior art is early membrane failure. Repeated flexing of the membrane along the line where it passes over the spacer-gasket interface is a primary cause of membrane failure. The failure of a single membrane necessitates a costly and time-consuming process comprising shutdown of the entire electrodialyzer and removal and replacement of the faulty membrane.
Several patents (e.g. U.S. Pat. Nos. 2,735,812; 2,848,402; 2,891,900; 2,948,668; 2,951,027; 3,761,386; 3,933,617; and 4,062,756) disclose membrane spacers which enhance fluid distribution over the membrane area and introduce turbulence, while preventing membrane collapse in the cell. However, these patents leave unsolved the problems of membrane flexing at the spacer-gasket interface and uniform fluid flow across the entire cell area.
U.S. Pat. No. 2,881,124 discloses a device comprising inserts for placing in the fluid inlet and outlet passages of an electrodialysis cell. These inserts support the adjacent membranes across the width of the passage while preventing them from being deformed into the passage. However, that invention makes no provision for preventing collapse of the membrane into the cell itself. The inserts of that invention constrain the fluid to a path which causes a substantial pressure drop and reduces the flow rate. Moreover, the fluid path does not vary across the width of the cell; thus, the flow will be greater near the inlet than near the edge of the cell. As discussed above, nonuniform fluid flow reduces cell efficiency.
U.S. Pat. Nos. 3,814,631 and 4,124,478 disclose apparatus which include fluid flow distributors; however, they have several drawbacks. They depend on a plurality of supply orifices to the cell rather than just one. Each orifice is provided with a plurality of channels to connect the orifice with the central cell area. Fabrication of these distributors is more complicated than fabrication of the distributors of the present invention and, moreover, the flow is less uniform since the channels at the center of the cell are identical to those near the periphery.